Full wireless earbuds are two earbuds that connect to each other wirelessly. Typically such full wireless earbuds follow a relay format or a sniff format. In the relay format, one earbud serves a master role, and the other earbud serves a slave role. The master earbud receives audio from a host, e.g., a mobile phone or other audio playback device, then relays the audio to the slave earbud. In the sniff format, one earbud is a primary earbud, and the other earbud is a secondary earbud. The primary earbud receives and acknowledges audio packets from the host, while the secondary earbud only receives audio, or “sniffs,” but never acknowledges the packet. If the secondary earbud loses packets, it will ask the primary earbud to re-transmit the packets.
Role switch between two earbuds happens when the roles of earbuds change. For example, for relay format earbuds, the master becomes the slave, and the slave becomes the master. For sniff format earbuds, the primary earbud becomes secondary, and the secondary earbud becomes primary. Role switch can happen if, for example, the slave earbud has better received signal strength, if the master earbud is running on lower battery than the slave, if the master earbud is taken out of the user's ear or put in a case, etc.
Wireless earbuds in relay format typically have a first asynchronous connection-less (ACL) link between the host device and the master earbud, where the master earbud is a Bluetooth slave in this ACL link. A second ACL link exists between the master earbud and the slave earbud. In this second ACL, the master earbud is the Bluetooth master, and the slave earbud is the Bluetooth slave.
In this format, it is very difficult to achieve seamless role switch. For example, most, if not all, profiles in the ACL between the master earbud and the host device need to be transferred to the slave earbud silently, so that after role switch the new master earbud can continue receiving audio packets. Further, a Bluetooth role switch is also required as the old master earbud becomes the new slave earbud, while the old slave earbud becomes new master earbud, because the master earbud should also be the Bluetooth master to be efficient at Bluetooth transmission.